Digital-to-analog converters (DACs) are widely used in electronic systems to receive a digital code and then generate an analog signal related to, or determined from, the received digital code. These types of converters are used most commonly as building blocks in metal oxide semiconductor (MOS) analog-to-digital conversion systems. For an N-bit voltage scaling DAC or resistor DAC (RDAC), the resistor string consists of 2N identical resistors connected in series, and is used as a potentiometer and/or a rheostat, among other applications, in which the voltage levels between successive resistors are sampled by means of binary switches.
In the rheostat mode, the RDAC behaves as a code-dependent resistor. Due to manufacturing process variability, the critical dimensions and process parameters can affect circuit performance and can result in die-to-die variation of absolute output resistance. One such effect is due to systematic offsets in the process parameters, such as doping, oxide thickness, lateral diffusion, vertical implant depth, critical dimension variation and so on, that can result in the variation of sheet resistivity of resistive materials. This variation in sheet resistivity can be as high as 30-50% of normal value. Thus, the VLSI based RDACs when used in rheostat mode cannot compete on performance with a discrete resistor used as rheostat which has about 1% variation over the normal value.
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.